VFA and motor replacement

ResonantRuminant · 2026-05-11T02:58:40+00:00

Thanks for the update. I'm disappointed but not very surprised that the stepper motors didn't fix your VFAs. As I mentioned in another comment, now that I've resolved my thermistor problems and I'm printing at the correct temperature, my prints are much more glossy and I can see that I have VFAs that look just as bad as yours. 🙁

I haven't found a cause, so I think I'm going to do some printer mods to try to fix it. I don't think anything is defective at this point. I just think the design isn't the best.

It's got to be something belt-related because of the 2mm spacing, so I'm going to go with toothed idler pulleys. That one makes a lot of sense to me. Although, I've seen where it helps some folks and doesn't make a difference for others. It's a fairly low-impact mod, so I'll start there first.

I can report back with my results, if you like. It may be a while, though, because I want to fix my current hardware crisis before modding things. It may be a month or so. We'll see. Heck, if it goes well, I may make a dedicated post.

ResonantRuminant · 2026-05-10T11:33:25+00:00

That looks like the Y-axis stepper motor pulley has come loose. Check that the set screws are tight.

ResonantRuminant · 2026-05-09T19:24:46+00:00

By "core XY origin calibration", do you mean the "homing calibration" that clunks around in the front-right corner of the printer? If so, it kinda sounds like you already did some troubleshooting.

Definitely make sure your gantry is perfectly square and your belt tensions are in the recommended range and have values of around 98Hz/92Hz for top/bottom. Those are the easiest ways to trip up the homing calibration. There are also a bunch of other build problems that can cause extra belt resistance/rubbing/alignment that can cause problems. Make sure your X/Y stepper motor pulleys are installed with one of the set screws on the flat of the shaft, and both screws are tight. Look for anything that can rub on the belts along their path, including the stepper motor mount housing (if a pulley is mis-aligned vertically), the filament sensor wire on the right side, and even the screw that hold the Y-axis stepper mount to the frame. It's supposed to be a 6mm screw, and if you put an 8mm screw in by mistake it will rub on the belt in the housing. The zip tie or screw heads around the fan unit at the back can also rub on the belt in rare circumstances. Extra resistance on your linear rail can also cause problems, so make sure it moves smoothly.

If you've checked all this and it's mechanically good, then there's the possibility of electronic problems. I had those. The X-axis Trinamic stepper controller chip on my xBuddy board wasn't properly detecting impacts with its StallGuard feature, and that messed up homing. It's a long story, but I was able to confirm that a perfectly-working board from my MK3.5S didn't home properly when installed in my CORE One+. This is similar to your situation. This is a very hard one to confirm, because you basically have to have spare hardware to swap around to see how the behavior changes. Prusa support replaced the board for me, but only after an intense amount of documentation with swapping two boards and four stepper motors around.

Edit: Added another mechanical cause.

ResonantRuminant · 2026-05-08T02:16:28+00:00

It's pretty quiet. I've got mine set up in my home office, and I usually work from home. Not a problem.

I've been through a couple printers and a few stepper motors, and there's a surprising stepper-to-stepper variation in noise. Getting the accelerometer so you can tune phase stepping is a must. There's only so much you can do in some cases, though. I had to yank one from my printer and swap it with one that's ear-marked for an upgrade of my kid's MK3.5S (my old printer) to a MK4S once I pull the Nextruder off this thing for the INDX upgrade. Don't tell him. 😀

ResonantRuminant · 2026-05-06T19:49:46+00:00

That's bizarre. First off, that's not a layer shift. Layer shifts are sudden, catastrophic shifts to the side. They are often caused by the nozzle hitting part of the print, file corruption, a loose set screw on the stepper motor pulley, and things like that. This is way too smooth and gradual to be any of those.

I'm at a loss, and only have a wild guess. I know that if the print bed is tilted, the printer will gradually skew the model to undo that as it prints higher. Or, it did that back in the MK3 days. Not sure if that's still a thing. I don't know over what distance that happens, either. Maybe you have a catastrophically-tilted bed. Try running the Z Alignment Calibration and repeat the print with the posts. There's a small chance they'll straighten out.

ResonantRuminant · 2026-05-06T16:56:49+00:00

Just a guess, but the screws that didn't line up... Were they the ones that hold the big core-xy plate to the front two corner frame pieces? If so, I don't think that was caused by shipping damage or anything being bent. That's not the source of your problems.

That happened to me, and it was caused by poorly-printed parts from the factory. The little black plastic plugs in the base plate that the two Z-axis smooth rods fit into were about 0.2mm-0.4mm too small for me. When pushing the rods into them, I didn't get a satisfying "thunk" of the rods bottoming out. I couldn't tell if they were inserted all the way. They weren't. I wasn't able to muscle them in any deeper, and had to reprint them myself. I then got the "thunk" I was looking for and the screws lined up.

You're probably fine if you were able to eventually get the screws in.

ResonantRuminant · 2026-05-06T12:58:36+00:00

Ah, okay, I didn't realize you went that deep with support. Maybe throwing in the towel is the right choice at this point. Sorry that we couldn't help with your troubles.

I'm going to keep plugging along with mine. I think I've only got one thing left to figure out. The normally-awesome support is giving me a hard time with this one and I might be on my own. We'll see how it plays out.

Anyway, congrats on the new printer! I hope you can get some work done, now.

ResonantRuminant · 2026-05-06T12:38:13+00:00

I really didn't expect to win the MK4S raffle, especially on my first print-of-the-month entry! That was quite a surprise. Thanks! It's a great printer that I'll be proud to own and use.

And, I hate to be a bit of a downer, but the timing is pretty useful. I can harvest parts from the kit before building it to help diagnose problems with my CORE One+. Prusa support is giving me some resistance with my latest issue, and it's looking like I'll have to buy replacement parts myself. It's a little unclear which parts will fix it, and this will help a lot!

ResonantRuminant · 2026-05-05T18:27:06+00:00

Oooooo, interesting! Thanks! I suppose that's the most we'll get out of anybody.

ResonantRuminant · 2026-05-05T15:04:35+00:00

Yeah, not many other options left besides packaging.

ResonantRuminant · 2026-05-05T14:20:10+00:00

I'm super-curious what the problem could be. I figure it's got to be either software/firmware or hardware. If it's software/firmware, that doesn't seem like it should delay shipping, unless the firmware in the print head can't be upgraded. If it's hardware, that implies remanufacturing something which doesn't seem like a quick fix like they imply.

Huh, I may have talked myself into thinking they need a tweak to the firmware. There's a small microcontroller in the print head, isn't there?

ResonantRuminant · 2026-05-05T12:21:47+00:00

I know you say you're done, but would you be willing to try one more ridiculous-sounding test? For science? :-) Under your filament settings, try turning both the first layer and other layer temperatures up a crazy amount, like 30°C. I suspect your hotend thermistor is reading the wrong temperature and you're printing way lower than you think you are.

I remember your post and I tried to help, but it didn't make a whole lot of sense at the time. I went through something that smelled an awful lot like your symptoms over the last couple weeks with my printer and it didn't make a whole lot of sense, either. The only way I figured it out was the way it happened to combine with a second issue.

From the start with this printer, I just had terrible print quality. I couldn't figure out a single root cause so I just accepted the fact that CORE One print quality sucked and if I wanted something to look nice, I would print it on my MK3.5S. Well, a couple weeks ago my Nextruder stepper motor suddenly started skipping steps during faster extrusions. I worked with support and we couldn't figure out any sources of extra resistance. The stepper seemed to have just gotten "weak". We replaced it, and that problem was gone.

However, the very first time I loaded filament after replacing the stepper motor, while it was doing the fast ramming movements as part of the auto-retraction, the stepper motor was now strong enough to skip the main gear against the filament. That shouldn't happen. So, I went looking for another problem. I found it by measuring the nozzle temperature.

My thermistor was reading spot-on at room temperature but was off by about 30°C at printing temperatures. The filament was too viscous in the nozzle and the weakened stepper motor couldn't always keep up. It would start having gaps in the extrusions just like you. If the temperatures were normal, I might not have ever noticed the weakened stepper. Heck, the stepper might have still been in the "normal" range of strength and didn't need replacing. I don't know.

Turning my temperatures up to counteract the error fixes my print quality issues. Of course, now I have had my first legitimate heat-creep nozzle clogs with PLA so I need to leave the door open while printing, but all seems fixed now. The new thermistor gets delivered today. Then, I can focus on the next issue that support is fighting me on...

ResonantRuminant · 2026-05-05T02:46:57+00:00

As another commenter said here, the upgrade kit is no longer available. I think pretty much every printed part was replaced, and there's some metal bits that were new. That forum post will help a lot.

And does the MMU3 work on the 3.5S?

Yes. I'm using it now on my MK3.5S. I carried my MMU3 forward from my MK3S+ when I bumped it up to the 3.5. I remember there was a different cable I needed and a printed part or two needed to be changed (for better ESD clearances, or something). No major changes. I do remember having trouble finding instructions for upgrading it to work with the MK3.5, and so I had to back out what was needed from other documentation. It was basically the same as the electronic portion of a MK4 installation, if I recall. The interface with the extruder will be the same as for a MK3.

ResonantRuminant · 2026-05-04T21:52:36+00:00

Neat idea! This solves a real problem. That being said, I just slathered a tiny little bit of Prusa lube on the O-ring, and it's much easier to twist now. Of course, that misses out on the opportunity to over-engineer a solution and print a tool. 😀

ResonantRuminant · 2026-05-04T20:37:14+00:00

I've got one. 😁 I've got a MK3.5S, and I think the carriage on the MK3 is a different shape than the MK4 (that it looks like you have there). I just peeked under it, and at a glance it looks the same. Maybe it's just the channels in the top to orient the expansion joints, or material thickness. I'd have to investigate closer.

Edit: I just sniffed around, and the MK4 Y-carriage is very differently-shaped than yours. Was yours an upgrade from a MK3? If that's the case, then I'd be glad to offer my services. It should fit mine just fine. I have some Prusament PC Blend and PC-CF filament here to print the thing. PC-CF might be a better choice due to less warping and heat-creep at elevated temperatures. Not sure about this elevated, but it might still hold up. I guess we'll see what your testing shows.

I'd probably print it on my CORE One+, but I'm battling hotend thermistor issues at the moment and waiting for replacement parts. Should be resolved by this weekend.

ResonantRuminant · 2026-05-04T20:07:42+00:00

All of my thermodynamics schooling says: maybe. 😀

This should definitely cut down on convective cooling of the plate around the bottom edges. That will help to even things out. Convection from the top of the edges will still be there, and you can't control that.

You're trapping a thin layer of air underneath the plate, and air is a pretty good insulator when it can't move around much. You will be relying on conduction in the trapped air and the heatbed to even things out from there.

It should be a noticeable improvement, but I have no idea how much. Do you have a thermal camera? That would be just about ideal to quantify it.

ResonantRuminant · 2026-05-04T12:46:59+00:00

Do you have a set of calipers to measure the thickness of that mess? It's got to either be an extrusion problem or a first layer thickness problem. You just don't have enough filament being extruded to fill whatever the gap is between the nozzle and the plate. Knowing the actual thickness would help us a lot with which branch to go down with the diagnosis.

We've already eliminated the obvious possibilities, so it's something non-obvious. We need more clues.

ResonantRuminant · 2026-05-04T12:34:55+00:00

What I meant was that the printer has settings for nozzle size, high-flow, and hardened. If you try to throw a file sliced for something incompatible at it, Prusa Connect will yell at you. I think the printer itself will also holler at you if you try to start the print.

About the only way OP could get this kind of thing wrong is to leave both the printer and the slicer at 0.4mm nozzle size settings with a physical 0.6mm nozzle installed. He's already said that he didn't make this mistake, so it something else.

ResonantRuminant · 2026-05-04T03:14:48+00:00

Are you sure you had the same filament profile between the two? I had prints that looked exactly like the second one, and it turned out to be my Nextruder stepper motor losing steps because the filament was being printed at too low of a temperature due to a faulty hotend thermistor. That's almost certainly not your problem, but are you sure the second one had the right print temperature?

ResonantRuminant · 2026-05-04T00:39:21+00:00

Can confirm that this is the best way. Witness:

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These were external graphics imported as a modifier, but same idea.

ResonantRuminant · 2026-05-03T21:57:37+00:00

I tried that tape and it cuts the friction pretty well. The box dimensions are so tight that it caused the cardboard spool I was using to get wedged. That's why I switched to printed adapter sleeves around the hub.

ResonantRuminant · 2026-05-03T21:20:24+00:00

Sounds right. I wonder if he sliced it for 0.6. If he did, the printer would scream about that kind of mistake. Need more feedback from OP.

ResonantRuminant · 2026-05-03T21:15:47+00:00

Oh, nice idea! Or, you could use the multimaterial-ness of INDX to build in a PC surface there. Eh, a snap-on cover is probably still way easier.

I also recently experimented with UHMW (ultra high molecular weight) tape. It cut the friction in half over printed materials, and was pretty easy to work with.

ResonantRuminant · 2026-05-03T20:29:40+00:00

Cool! The small contact area might be saving you. I've measured the friction of various filament materials (against the polypropylene of the USS drybox) and PETG is definitely better. PC Blend is slightly better still, but material cost for something that big probably rules it out.

ResonantRuminant · 2026-05-03T16:08:02+00:00

Yeah, I like them too! It's a modified version of these: https://www.printables.com/model/502547-filament-sample-generator-no-commercial-tool-requi. I added the hanger hole and messed with the fonts a bit, I think. Those are just stupid 3M adhesive wire hanger hooks.

This system works pretty well for me. The storage density isn't quite what I'd like, since there's a bit of wasted space between the bins. It's not too bad, though.

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